This commit removes the various compiler-specific JUCE_DEPRECATED macros and replaces them with C++14's deprecated attribute. It also removes the JUCE_CATCH_DEPRECATED_CODE_MISUSE flag as we can rely on the override specifier catching usage of these old virtual methods, and tidies up the DOXYGEN preprocessor checks as they were inconsistent across the codebase.

Previously, we were always adding a #version 150 directive to shaders
when the GLSL version was higher than 1.2, which could cause compilation
to fail on platforms with GLSL versions between 1.2 and 1.5.